Abstract:
Continental margins at low latitudes typically receive sediments derived from erosion of adjacent landmasses or from carbonate reefs that form from the organic and chemical precipitation of calcium carbonate. Because the development of carbonate reefs and platforms generally requires low volumes of siliciclastic input, these two endmenber depositional settings are generally considered to be mutually exclusive. However, there are also margins that develop in mixed carbonate/siliclastic depositional environments. Such mixed settings can display distinct alternations between predominantly carbonate-rich and predominantly siliciclastic-rich sedimentary units that are inferred to be a response to sea level and climate change, tectonism and sediment supply over time. Facies developments in a setting of mixed carbonate-siliciclastic sediments that grew at the forereef margin of the Great Barrier Reef (GBR) in the Pleistocene to Recent (1.5-0 Ma) are examined. The transect of ODP sites 819, 820 and 821 is located immediately seaward of the central GBR near Grafton Passage, east-northeast of Cairns, Queensland, Australia. Sedimentology and sedimentary petrology are combined with seismic reflection data, wireline/physical properties, core data, and published oxygen isotope data and newly developed concepts of sequence stratigraphic analysis. Statistical analysis of oxygen isotope data are also used to investigate the periodicities of sediment accumulation, and the potential of climate modulation of sedimentation. Depositional sequences at the GBR margin contain thin, well-defined condensed sections that provide important clues to its depositional and climatic history. This study also investigates the relationships of condensed sections to sequence stratigraphic architecture and to cyclic sedimentation. Modern studies of past variations in sea level are usually from two distinct lines of research: seismic sequence stratigraphy (Vail et al., 1977a, 1991), and oxygen isotope stratigraphy (Shackleton and Opdyke, 1973; Hays et al., 1976; Imbrie et al, 1984). Because of high sedimentation rates along the GBR margin, the average duration of many of the sequences approximates third- (0.5-2 my), fourth- (0.08-0.5 my), and fifth-order (0.01-0.08 my) cycles of sea-level change. Their duration may also approximate cyclicity in the range deduced from the oxygen isotope record. The cyclic nature of the depositional sequences at the GBR is further investigated in an attempt to determine if there is a link between them and Milankovitch orbital forcing parameters. My results suggest orbital forcing may influence and may regulate depositional processes at the GBR through climate and eustatic sea-level change, and variations in rates of terrigenous influx, carbonate shedding, and pelagic sedimentation. The oxygen isotope record in marine carbonates, when integrated with sequence stratigraphic interpretations, has the potential to enhance detection of stratal surfaces that may not be well constrained by biostratigraphic data. It appears that the sequence stratigraphic interpretation of the forereef deposits at the GBR transect closely matches significant aspects of the climate record for at least the past approximately 650 ky as revealed in the oxygen isotope signal at Site 820. Sequence boundaries and condensed sections correlate well with the most extreme glacial to interglacial transitions. On the basis of oxygen isotope (Peerdeman et al., 1993), magnetic susceptibility and sedimentologic data, the upper 150 mbsf of Site 820 can be divided into a lower section (70-150 mbsf) characterized by a relatively high-frequency signal, and an upper section (0-70 mbsf) characterized by a relatively low-frequency signal. Spaectral analysis of the oxygen isotope signal from a nearly complete Pliocene and Quaternary sequence in the Gulf of Mexico indicates that the 41 ky period was dominant during the early Pleistocene and the entire Pliocene and the 100 ky period has dominated during the last approximately 500 ky. The 500 ky transition corresponds with Stage 13, which is at approximately 90 mbsf at Site 820. There is a high probability that stratigraphic gaps exist in the data that are represented by depositional hiatuses or submarine erosion. Sedimentation associated with sea level lowstands bypasses the shelf transect and is deposited basinward of Site 819 in deeper waters. In is quite possible that submarine erosion occurs at the transect during these times as there are time discontinuities in the oxygen isotope record. Although this record appears incomplete, it provides the highest-resolution chronology at the GBR transect to date. Spectral analyses of the oxygen isotope record reveal power spectra that suggest that Milankovitch cyclicity is present, but much of the signal does not coincide closely with the three orbital periods.